1. The ganglioside GM1, the receptor for cholera toxin (CT), can also function as a receptor for E. coli heat-labile enterotoxin. Thus, these two pathogenic toxins, which activate intestinal mucosal adenylate cyclase by the same mechanism of ADP-ribosylation, appear to utilize the same receptor. 2. Although gangliosides function as receptors for bacterial toxins, they are not involved in the binding and action of glycopeptide hormones such as LH and hCG. Murine Leydig tumor cells contain gangliosides and bind and respond to hCG and CT. Using various techniques, GM1 was shown to be the CT receptor in these cells whereas the hCG receptor was found to be a glycoprotein. In addition, gangliosides were not required for hormone action. 3. Gangliosides are synthesized inside the cell and then transported to the plasma membrane. Transport is temperature dependent but not blocked by inhibitors of protein synthesis, cytoskeletal assembly or energy metabolism. Synthesis appears to occur at two separate sites. Glucosylceramide is formed to the rough endoplasmic reticulum whereas further glycosylation occurs in the Golgi apparatus. Several drugs including monensin, an ionophore, block the conversion of glucosylceramide to more complex glycolipids by preventing its transport to the second glycosylation site in the Golgi.